Conventionally, a vehicle-mounted device can use DSRC (Dedicated Short Range Communication) to perform short range wireless communication with a roadside apparatus installed on a road, and can be provided with information by speech from a center apparatus via the roadside apparatus. In the communication using the DSRC, only while a vehicle exists in a communication area of the roadside apparatus, two-way communication between the vehicle-mounted device on the vehicle and the roadside apparatus is enabled, and in this period, the center apparatus distributes various information for supporting safety, such as traffic situation information, disaster point information, road freezing point information, curve information, and junction point information, on a lane ahead on which the vehicle travels, via the roadside apparatus.
For example, a roadside apparatus installed near an inbound lane emits a DSRC radial electric wave to a vehicle-mounted device mounted on a vehicle traveling on the inbound lane, and distributes information related to the inbound lane. Moreover, a roadside apparatus installed near an outbound lane emits a DSRC radial electric wave to a vehicle-mounted device mounted on a vehicle traveling on the outbound lane, and distributes information related to the outbound lane.
However, the radial electric wave may be reflected on the vehicle or by a construction such as a building, and the reflected electric wave may be directly leaked to an adjacent lane. In this case, an utterance type vehicle-mounted device not having a navigation function cannot discriminate which direction the vehicle itself is traveling in, and thus may erroneously receive information transmitted to a vehicle traveling on an opposite lane, and may erroneously perform speech output of information related to the road related to the opposite lane. For example, while a driver is traveling on the inbound lane, an electric wave reflected from an adjacent outbound lane may be received, the information related to the outbound lane may be outputted, and the driver may be confused.
Consequently, in the communication using the DSRC, a technique has been proposed in which erroneous communication is prevented from being performed due to the electric wave leaked to the adjacent lane. For example, Patent Document 1 describes a technique for causing an electric wave reflected by a road surface to be incident on an electric wave absorber to absorb the electric wave, and preventing defects caused by an unnecessary leakage electric wave.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-237719